Heating appliance backdrafting:
This article discusses the causes and cures of back drafting and flue gas spillage hazards in tight houses.
We explain the difference between flue gas spillage and back-drafting. While the trend is toward power-vented appliances, most furnaces, boilers, and water heaters still use atmospheric or “natural” venting. Atmospheric venting relies on the natural buoyancy of warm air in the flue or chimney to carry exhaust gases from the home.
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The strength of the draft depends on the temperature difference between the flue gases and outside air, the height of the chimney, and the indoor air pressure.
In newer, tighter houses, significant negative pressures can be generated by kitchen and bath exhaust fans, gas dryers, and unbalanced air flows in the home’s air distribution system.
Unbalanced pressures can also be caused by leaks in return ductwork, by the use of building cavities as ducts, or by the simple closing of bedroom doors in homes with a central return register. Leaky return duct- work in a basement may be enough to backdraft a water heater or furnace.
See COMBUSTION AIR for TIGHT BUILDINGS for details on how to provide adequate combustion air in tight buildings.
Spillage vs. backdrafting. If a naturally vented appliance lies in an area of the house with strong enough depressurization, the flue gases will spill into the home. When the flow reversal lasts for 30 seconds to a minute, it is called spillage; longer sustained spills are called backdrafting, a far more serious condition.
If the gas-log fireplace shown at left is not properly vented, spillage of combustion gases including possibly carbon monoxide into the building interior may be continuous. In tight quarters there is also the risk of oxygen depletion.
Watch out: Both of these are potentially fatal hazards.
While the manufacturer of the gas log shown in our photograph advertised that the burner includes a safety device that would shut off the fire in the event of dangerous oxygen depletion, we found that we were unable to operate this appliance without setting off the carbon monoxide detector alarm in the room, and the alarm continued to sound even when the detector was placed in an adjoining hallway.
Continuing from from Best Practices Guide to Residential Construction:
Once backdrafting begins and the flue gets cold, it may be sustained for a long time. Research has shown that negative pressure of as little as 5 Pascals (Pa) creates a risk of backdrafting with naturally vented boilers and furnaces. Numerous studies have documented the prevalence of high negative pressures and frequent spillage in new homes built to current codes but not intentionally built airtight.
If the heating equipment is well adjusted and has adequate combustion air, the flue gases will contain primarily water vapor and carbon dioxide, along with nitrous and sulfur oxides, and particulates.
If the burner is malfunctioning for any reason, it may put out large quantities of carbon monoxide and turn a backdrafting situation deadly. Fireplaces and poorly sealed woodstoves are most likely to reverse flow late at night when the fire is smoldering, producing a weak draft and high levels of CO.
Guide to Preventing Backdrafting at Heating Equipment, Woodstoves, & Fireplaces
There are three key elements to preventing backdrafting:
Watch out: as we discuss also
When the supply of cooled or heated air to occupied building spaces is inadequate and the cause is suspected to be inadequate return-air in the system duct design, an amateur retrofit "repair" for this condition is to simply cut additional return air openings in the return duct.
But if those openings are cut too close to a heating appliance, especially gas fired heating appliances, the in-draft created by the return air opening cut close to the gas burner can interfere with adequate combustion air supply to the burner, causing dangerous or even fatal carbon monoxide production, and backdrafting into the occupied space.
See COMBUSTION AIR for TIGHT BUILDINGS for additional details. - Ed.
Chimney Problems Related to Backdrafting
A chimney or flue that is too large, too small, or blocked by a bird’s nest or loose brick will not draw properly and will be prone to spillage problems. Uninsulated chimneys on outside walls are also prone to poor draft and to condensation problems that can deteriorate flue materials. These problems should be fixed first before addressing problems inside the house.
At CHIMNEY INSPECTION DIAGNOSIS REPAIR we discuss chimney inspection and diagnosis including unsafe venting and fire hazards.
Heating systems with fan-powered exhaust systems can withstand higher negative pressures than natural-draft appliances. Some types of fan-powered systems are much better than others, however. In order of effectiveness, the choices are:
By comparison, an atmospherically vented furnace can backdraft with as little as 5 Pa of negative pressure, and a gas water heater will have spillage at 2 or 3 Pa. Fireplaces can start having problems at about 3 Pa. Canadian codes limit negative pressures in homes with atmospherically vented equipment to 5 Pa. U.S. codes do not currently address the issue.
Guide to Reducing Building Depressurization to Assure Safe Heating Equipment Venting
To keep indoor depressurization to a minimum, do not oversize bathroom and kitchen fans, and avoid the use of downdraft and island fans, which can draw 600 cfm or more. If large fans must be used, they should be interlocked with a supply fan to provide makeup air.
Or see VENTILATION, BALANCED
Canadian requirement for fan-supplied make-up air in buildings
Canada’s 1995 National Building Code requires that in homes with fuel- burning appliances vented through a chimney, any exhaust fan with a net capacity greater than 160 cfm must have fan- supplied makeup air.
The makeup air fan should be sized to reduce the net exhaust rate to no more than 160 cfm and can be delivered to an adjacent room or through the forced-air distribution system. For example, a 300 cfm exhaust fan should have at least 140 cfm (300 minus 160) of makeup air.
How much an exhaust fan will depressurize a house depends on the tightness of the house. A 1993 study of several newly built energy-efficient homes in Minnesota found that exhaust airflows of 300 to 550 cfm depressurized the homes to 5 Pa, the level at which natural-draft appliances start having spillage problems.
Other studies indicate that a 600-cfm exhaust fan can produce negative pressures from 3 to over 20 Pa, depending on house tightness. Without an adequate source of makeup air, a fan this size (or a combination of exhaust fans running at the same time) will pull air from the path of least resistance—often a nearby chimney or flue. Unless makeup air is provided, exhaust fans of this strength should not be used in homes with chimneys.
How to Check for Dangerous Chimney & Flue Backdrafting by Performing a "Worst Case" Test
In homes with the potential for back- drafting, a simple test can be conducted to determine the likelihood of problems:
Perform the test with the air handler both on and off, since unbalanced airflows can be a significant factor. If smoke spills into the room for more than 30 seconds at any combustion appliance, the home has a potential backdrafting problem that requires attention.
A more scientific procedure for determining backdrafting potential, using a pressure gauge, can be found in Step 7 of the “Recommended Procedures for Safety Inspection” in Appendix H of the National Fuel Gas Code.
Traditional open fireplaces and older leaky woodstoves burn very inefficiently and produce hundreds of chemical compounds, including carbon monoxide, organic gases, particulates, and some of the same cancer-causing agents found in tobacco smoke. Minor spillage of these pollutants occurs regularly, primarily when starting or stoking the fire. However, the larger concern is when the fire smolders late at night, producing high levels of CO and a weak draft. Backdrafting at this time can be dangerous or even fatal.
Another problem, particularly with fireplaces, is created when the fire is roaring and drawing up to 400 cfm of combustion air. At this point, its voracious appetite for air can cause backdrafting in other combustion appliances such as a gas water heater. Also, the need to reheat all the makeup air drags down the fireplace’s heating efficiency to less than 15% and, if the fireplace is allowed to smolder all night, it becomes a net heat loser.
Woodstove efficiency has improved dramatically in response to EPA emissions standards (begun in 1988 and updated in 1990), which apply to most freestanding wood stoves and to fireplace inserts with air-supply controls and tight-fitting doors. To meet these standards, manufacturers use either a catalytic converter, similar to the ones used in cars, or a reengineered firebox.
The new fireboxes have primary and secondary combustion zones capable of reaching system efficiencies of 60% or more and reducing combustion air intake to as little as 10 cfm. If installed with an outdoor air supply, these can be successfully de- coupled from household air pressures.
While many fireplaces are fitted with glass doors, and some have outside air intakes, nearly all of the glass doors leak air. Even with low levels of depressurization, these fireplaces can still backdraft, and the fireplace’s outdoor air supply might become the makeup air for the kitchen range hood or other exhaust fans, drawing fireplace fumes along with it. The best solution is an airtight fireplace insert.
To minimize pollution, indoors and outside, from wood-burning appliances
-- Adapted with permission from Best Practices Guide to Residential Construction.
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